To support comparing CPUID data update with what's already set for a vCPU
all mangling needs to happen in __kvm_update_cpuid_runtime(), before
'vcpu->arch.cpuid_entries' is updated. CPUID.(EAX=0x12,ECX=1) is currently
being mangled in kvm_vcpu_after_set_cpuid(), move it to
__kvm_update_cpuid_runtime(). Split off cpuid_get_supported_xcr0() helper
as 'vcpu->arch.guest_supported_xcr0' update needs (logically) to stay in
kvm_vcpu_after_set_cpuid().
No functional change intended.